Biomechanical evaluation of total disc arthroplasty: an in-vitro human cadaveric model

Abstract

Purpose of study: This in-vitro biomechanical study was undertaken to compare the multidirectional flexibility properties and map the instantaneous axis of rotation (IAR) of total disc arthroplasty versus conventional threaded fusion cages and cages augmented with transpedicular fixation for single-level spinal reconstruction.Methods used: A total of eight human cadaveric lumbosacral spines (L3 to sacrum) were used in this investigation and biomechanically evaluated under the following L4–L5 reconstruction conditions: 1) intact spine; 2) SB Charitè Disc Prosthesis; 3) BAK cages; 4) BAK cages + ISOLA pedicle screw/rod fixation (anteroposterior). The superior (L3–L4) and inferior (L5–S1) intervertebral levels remained unconstrained to quantify adjacent level properties. Multidirectional flexibility testing included pure, unconstrained moments (±8 Nm) in axial rotation, flexion-extension and lateral bending, with quantification of the operative and adjacent level range of motion (ROM) and neutral zone (NZ), which were normalized to the intact condition. Stepwise flexion-extension radiographs were digitized and used to calculate the intervertebral IAR for the operative and adjacent levels.of findings: The SB Charitè prosthesis indicated an average percentage increase in axial rotation ROM by 44% compared with the intact condition (p<.05), whereas the BAK and anteroposterior reconstructions decreased ROM by 29% and 80%, respectively (p<.05). The SB Charitè was significantly different from BAK and combined anteroposterior reconstructions (p<.05). Flexion-extension indicated a minor increase in ROM for the SB Charitè (3%) versus the intact disc (p>.05), whereas the BAK and anteroposterior stabilization groups resulted in significant decreases in ROM (BAK=57%, anteroposterior=93%) (p<.01) when compared with the intact and SB Charitè conditions. Based on flexion-extension radiographs, the intervertebral centers of rotation were in the posterior 33% of the operative intervertebral disc only for the SB Charitè reconstruction, with definitive evidence of physiologic intervertebral translation (6 mm).Relationship between findings and existing knowledge: The SB Charitè restored motion to the level of the intact segment in flexion-extension and lateral bending and increased motion in axial rotation. The anterior annular resection necessary for device implantation and unconstrained design of the prosthesis account for this change in rotation. The normal flexion-extension axis of rotation of a motion segment is an ellipse rather than a single point. Only disk replacement rather than pedicle instrumentation or BAK interbody instrumentation preserves the normal mapping of segmental motion at the operative level and the adjacent vertebral spinal levels.Overall significance of findings: Total disk replacement is currently the only method of restoring mobility to the degenerative lumbar disc. This serves as the first comprehensive biomechanical study comparing the operative level multidirectional flexibility characteristics between total disc arthroplasty, interbody arthrodesis cages and pedicle screw spinal instrumentation.Disclosures: Device or drug: SB Charitè Prosthesis. Status: Investigational. Device or drug: BAK/L cages. Status: Approved. Device or drug: Isola Pedicle screws and rods. Status: approved.Conflict of interest: Ryan W. Cunningham, grant research support: Waldemar Link Inc.